Plasma mass spectrometer

Abstract

A plasma mass spectrometer generates an aerosol of a sample, by dissolving the sample in a liquid solvent, and spraying the liquid into a spray chamber via a nebulizer. The solvent condenses in the spray chamber and an aerosol of the sample then passes through a supply tube to a plasma torch. Microwave power from an output on the plasma torch converts the aerosol to a plasma, and the plasma passes to an analyzer. If microwave radiation reached the spray chamber, it would cause heating of the solvent which may evaporate it so that the solvent would be present in the plasma. Therefore, there is a wall between the spray chamber and the plasma torch and microwave output which blocks such microwave radiation.

What is claimed is: 
     
       1. A plasma mass spectrometer comprising: generating means for generating an aerosol of a sample;   a plasma torch for receiving the aerosol from said generating means;   microwave output means mounted in proximity to said plasma torch for supplying microwave frequency power to said plasma torch, thereby to ionize said aerosol in said plasma torch to form a plasma of ions; and   means for analyzing said plasma by mass spectrometry;   wherein a shielding member is provided between said generating means and said microwave output means for shielding said generating means from microwave frequency power applied by said microwave output means to said plasma torch.   
     
     
       2. A plasma mass spectrometer according to claim 1, wherein said shielding member is a wall having an opening therein for passage of said aerosol from said generating means to said plasma torch, said opening having a diameter D such that ##EQU3## where λ c  is the wavelength of the high frequency power; ε R  is the specific dielectric constant of the transmission medium of said microwave frequency power between said microwave output means and said generating means; and   A≈1.7     
     
     
       3. A plasma mass spectrometer according to claim 2, having a casing around said microwave output means and said plasma torch, said wall being part of said casing. 
     
     
       4. A plasma mass spectrometer according to claim 1, having an aerosol supply tube extending through said shielding member from said generating means to said plasma torch. 
     
     
       5. A plasma mass spectrometer according to claim 1, wherein said generating means includes a nebulizer and a spray chamber. 
     
     
       6. A plasma mass spectrometer according to claim 1, wherein said microwave output means is a microwave cavity connected to said plasma torch, and said plasma mass spectrometer further includes a microwave source and a waveguide connecting said microwave source to said microwave cavity. 
     
     
       7. A plasma mass spectrometer according to claim 6, wherein said shielding member is formed in a casing which encloses said plasma torch, and wherein said microwave source is located outside said casing. 
     
     
       8. A plasma mass spectrometer according to claim 6, wherein said generating means, said plasma torch and said microwave cavity are enclosed in a casing, and wherein said shielding member is provided as a wall within said casing between said generating means and said microwave cavity. 
     
     
       9. A plasma mass spectrometer according to claim 8, wherein said microwave source is located outside said casing. 
     
     
       10. A plasma mass spectrometer according to claim 1, wherein said shielding member is made of metal.